In recent years, as a processing laser, a fiber laser made of a fiber having Yb incorporated has been developed as a substitute for a solid YAG laser. The fiber laser is characterized in that the beam quality is good since the light propagation mode is limited, and heat dissipation is excellent by a fine fiber, whereby no cooling is required.
As the fiber having Yb incorporated, one containing quartz glass as the base is usually employed. The absorption of Yb3+ observed in the vicinity of 975 nm of a Yb-incorporated single mode fiber using quartz as the base, is usually from about 0.8 to 3.5 dB/cm. Further, the absorption of Yb3+ in glass has a sharp peak in the vicinity of 975 nm and has another absorption peak in the vicinity of 915 nm on the shorter wavelength side thereof (Non-Patent Document 1). As the excitation wavelength for a Yb-incorporated fiber, 975 nm or 915 nm, or less than 975 nm and more than 915 nm, is usually employed.